Method of protection of d.c.transmission lines and a device for accomplishing same

ABSTRACT

A method of protecting D.C. transmission lines, based on shaping a signal proportional to a time derivative of the varying voltage on a line being protected. A reference signal of a preset duration is produced at the moment of emergence of the signal being shaped, the duration of said shaped signal being compared against the duration of the reference signal within a time interval until said derivative changes its sign. The duration of the signal being shaped within the preset time interval smaller than that of the reference signal is indicative of a likely short-circuit in the very line being protected, while the duration of the signal being shaped within the same time interval longer than that of the reference signal is indicative of a likely fault at the convertor substations of a D.C. transmission line. A device for the realization of said method comprises: a selection unit and a protection unit controlled by the former; an input of the selection unit is connected to the line being protected via a voltage divider, an output of said selection unit being, at the same time, one of the inputs of the protection unit; the rest of the inputs of said protection unit are connected to the D.C. line under protection via a D.C. transformer and the voltage divider, correspondingly. The selection unit is fashioned as series-connected differentiator for shaping a signal proportional to the time derivative of the voltage in the beginning of the very protected line, an amplifier and a pulse maximum duration selector, while the protection unit comprises: a minimum voltage member, a maximum current member and three logical circuits: one is the so-called &#39;&#39;&#39;&#39;prohibitive&#39;&#39;&#39;&#39; and the other two &#39;&#39;&#39;&#39;matching&#39;&#39;&#39;&#39; circuits.

United States Patent [191 Maranchak et al.

[ 11 3,745,417 1 July 10, 1973 METHOD OF PROTECTION OF D.C.

TRANSMISSION LINES AND A DEVICE FOR ACCOMPLISHING SAME [76] Inventors: Vasily Makarovich Maranchak,

Lefortovsky val, 716, korpus 9, kv. 12; Vladimir Nikolaevich Novella, Utrennyaga ul. l0, korpus 2, kv. 43; Stanislav Petrovich Veisky, ul. Lobanova 4 kv. 36, all of Moscow, USSR [22] Filed: Sept. 7, 1972 [21] Appl. No.: 287,078

Related US. Application Data [63] Continuation-impart of Ser. No. 29,605, April 17,

1970, abandoned.

Primary Examiner.l. D. Miller Assistant Examinerl'iarvey Fendelman AttorneyEric l-l. Waters, John G. Schwartz and .1. Harold Nissen et al.

[5 7] ABSTRACT A method of protecting D.C. transmission lines, based on shaping a signal proportional to a time derivative of the varying voltage on a line being protected. A reference signal of a preset duration is produced at the moment of emergence of the signal being shaped, the duration of said shaped signal being compared against the duration of the reference signal within a time interval until said derivative changes its sign. The duration of the signal being shaped within the preset time interval smaller than that of the reference signal is indicative of a likely short-circuit in the very line being protected, while the duration of the signal being shaped within the same time interval longer than that of the reference signal is indicative of a likely fault at the convertor substations of a D.C. transmission line. A device for the realization of said method comprises: a selection unit and a protection unit controlled by the former; an input of the selection unit is connected to the line being protected via a voltage divider, an output of said selection unit being, at the same time, one of the inputs of the protection unit; the rest of the inputs of said protection unit are connected to the D.C. line under protection via a D.C. transformer and the voltage divider, correspondingly. The selection unit is fashioned as seriesconnected difierentiator for shaping a signal proportional to the time derivative of the voltage in the beginning of the very protected line, an amplifier and a pulse maximum duration selector, while the protection unit comprises: a minimum voltage member, a maximum current member and three logical circuits: one is the so-called prohibitive and the other two matching circuits.

2 Claims, 4 Drawing Figures PATENTEBJUL 1 mm- 3,745.41?

sum 1 or 3 FIG.1

PATENTEQJULI DISH SHEET 3 BF 3 METHOD OF PROTECTION OF D.C. TRANSMISSION LINES AND A DEVICE FOR ACCOMPLISHING SAME The present invention is a continuation-impart of application Ser. No. 29,605 of Apr. 17, 1970, now abandoned.

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to relay protection of power transmission systems and more particularly to methods of protecting D.C. transmission lines and to devices for the realization thereof.

Known in the art is the method of a short-circuit protection of D.C. transmission lines, whereby the selectivity of the protection is achieved by tracing a difference between the time derivatives of the voltage on the very line being protected.

This method covers short-circuits on the very line being protected as well as faults occurring at its convertor substations, one of which, i.e., the rectifying substation, is connected in the beginning of said line and the other, i.e., the invertor one, in the end of said line under protection, the time derivative of the voltage on said line in case of a short-circuit thereon being, as a rule, substantially greater than that in case of any fault at the convertor substations (cf. USSR Inventors Certificate No. 134749, cl. 21c, 68/50, lPCl-l 02).

Known also are devices for protecting D.C. transmission lines against short-circuiting.

Some of them are sensitive to a certain time derivative of the voltage on the very line being protected and are triggered only in case of short-circuits on said lines. They comprise a differentiator connected to the very line being protected via a voltage divider, an actuator coupled to the output of said differentiator, a signal emerging at the output of said actuator only in case of short-circuits on the very line being protected and being made use of for disconnection of a rectifying substation of said D.C. transmission line (cf. British Pat. No. 1,120,460, IPC H 02h3/24) Also known are the other types of devices for protecting D.C. transmission lines against short-circuiting (cf. US. Pat. No. 3, 444, 433 lPC H 02 h 1/04), which are sensitive to variations in the voltage on the very line being protected.

At the same time, said known devices may be also sensitive to the value of the current on said line, a precondition for triggering said devices in this case being the presence of current on said line, whose value should be greater than a certain minimum value.

Such known devices comprise a minimum voltage member, a maximum current member, a time-lag member, and an actuator. The minimum voltage member is connected to the line being protected via a voltage divider and the maximum current member via a D.C. transformer. Said devices are triggered both in case of short-circuits on the very line being protected and in case of such faults at an invertor substation of said line, as, as in the case with the above short-circuits on the line being protected, can be eliminated only by disconnecting a rectifying substation provided in the beginning of said line. A signal emerging at the output of the actuator of the device of such type in case of abovementioned faults is made use of for disconnecting the rectifying substation.

The third type of a device for protecting D.C. trans mission lines is fashioned as a combination of the first two types described hereinabove.

Such is, for instance, a device described in the Swedish Patent No. 303,537 lPC H01 h 83/12.

This device has two units one of which is sensitive to a certain time derivative of the voltage on a line being protected and can be called a selection unit and the other one, being sensitive to the voltage on said line, can be called a protection unit.

The selection unit is connected to the very line being protected via a voltage divider and comprises a differentiator, connected to the output of which is an actuator. The protection unit has a minimum voltage member, delay element, and an actuator, the minimum voltage member being connected, via said voltage divider to the line being protected in the beginning of said line. The outputs of the actuators of the selection and protection units are combined to form a common output of the device. A signal emerges at said output of the device both in case of short-circuits on the line being protected and of such faults at an invertor substation, as can be eliminated only after disconnecting a rectifying substation provided in the beginning of said line, said signal being made use of for disconnecting said rectifying substation.

A disadvantage of the known method is a dependence of said time derivative of the voltage upon the working voltage on the very line being protected.

At the same time, said known devices suffer from an inadequate sensitivity when provided on lines of great extension as well as on aerial ways with a cable link, as sensitivity to short-circuits on the very line protected drops due to a reduced derivative being measured as response to a reduction in the working voltage on said line.

It is an object of the present invention to provide a method of protecting D.C. transmission lines, as would help preclude the effect by the working voltage on a line being protected on the measured parameter of the time derivative of the voltage on said line.

Another object of the invention is to provide a device for protecting D.C. transmission lines, whose high sensitivity to short-circuits on the line being protected and high speed operation could be ensured at any possible working voltage on said line and on its remote parts.

One more object of the invention is to secure high parameters for the device when operating both on aerial and cable D.C. transmission lines and on an aerial line with a cable link.

These and other objects of the invention have been accomplished by shaping, under the method of protecting D.C. transmission lines, a signal proportional to a time derivative of the voltage being varied on said line under protection; producing a reference signal at the moment of the emergence of said signal being shaped, the duration of said reference signal being selected, ac cording to the invention, longer than the maximum duration of said signal being shaped within the time interv val, until said time derivative changes its'sign due to short-circuits on the line being protected and shorter than the minimum duration of said signal being shaped within said time interval in case of faults at convertor substations on said line; and then comparing the duration of said signal being shaped within the time interval, until said time derivative changes its sign, with the duration of said reference signal, the duration of said signal being shaped, shorter than the duration of said reference signal within said time interval, being indicative of a likely short-circuit on the very line being protected, while the duration of said signal being shaped, longer than the duration of said reference signal within the same time interval, implying a likely fault at the convertor substations of said D.C. transmission line. The object of the invention is also achieved by provision that, in a device for the realization of the proposed method, comprising a selection unit connected to a line under protection via a voltage divider and a protection unit coupled with said selection unit, with a voltage divider and with a D.C. transformer, all being connected to said line, the selection unit is fashioned as seriesconnected differentiator, amplifier, and pulse maximum duration selector, plus an actuator whose output is connected to one of the inputs of the protection unit. The pulse maximum duration selector is made up of a reference signal presetter, a flip-flop and a logical prohibitive circuit.

An advantage of the proposed invention, as compared with the known devices, is a high sensitivity to short-circuits on a line being protected, which is free from the effects of variations in the working voltage on said line.

Another advantage of the proposed invention is that it can be equally provided on aerial and cable D.C. transmission lines and on said aerial lines with a cable link.

One more advantage of the given invention, as compared with the known devices, is its high speed as response to short-circuits on the line under protection, and on its remote parts.

The invention will be more apparent from the following description of its exemplary embodiment, reference being had to the appended drawings, whereon:

FIG. 1a shows schematically a graph of variations in the voltage at the beginning of a D.C. transmission line being protected in case of a short-circuit at the end of it;

FIG. 2b is a graph of variations in the time derivative of the voltage on a line being protected within a time interval, until said derivative changes its sign in case of a short-circuit at the end of said line, according to the invention;

FIG. 2a is a graph of variations in the voltage at the beginning of a D.C. transmission line being protected in case of a short-circuit beyond a surge suppressor from the side of the invertor substation;

FIG. 2b is a graph of variations in the time derivative of the voltage on theline being protected within a time interval until said derivative changes its sign as response to a short-circuit beyond the surge suppressor at the invertor substation, according to the invention;

FIG. 3 shows a block circuit diagram of a device for protecting D.C. transmission lines against shortcircuits, according to the invention;

FIG. 4 shows an exemplary embodiment of the functional schematic diagram of a device for protecting D.C. transmission lines against short-circuits, according to the invention;

A method of protecting D.C. transmission lines consists in the following.

It is well-known that the speed of variations in the voltage of a D.C. transmission line being protected depends, in the course of a first reduction in said voltage, upon a place where a fault occurs.

In case of short-circuits on the very line being protected, the voltage U, (FIG. la) varies, in the beginning of said line during its first drop limited by points A and B,, with a speed greater than does same voltage U, in the beginning of the line during its drop limited by points A and B in the event of malfunction with one of the convertor substations, the time interval corresponding to the drop AB, (FIG. 1a) being essentially smaller than the time interval corresponding to the drop AB (FIG. 2a).

The difference in the durations of said time intervals corresponding to the drops AB, and AB underlies said method of protecting D.C. transmission lines.

To establish said time intervals, it is necessary to shape a signal proportional to, say, the first derivative dU,/dt (FIGS. 1b, 2b) of the voltage U, (FIGS. la and 2a) in the beginning of the very line being protected. It turns out that the duration of a time interval corresponding to the drop AB, (FIG. la) in case of a shortcircuit on the very line being protected is equal to A t, (FIG. 1b) within the limits of which said derivative dU,/dt has not yet changed its sign, said time interval A 1, being measured from the moment of the beginning of a voltage drop U, (FIG. 1a, point A) up to the moment corresponding to a bend point of said voltage drop (FIG. la, point B,). Similarly, the duration of a time interval corresponding to a voltage drop Ab, (FIG. 2a) in case of a fault at a convertor substation is 2to A t (FIG. 2b) within the limits of which said derivative dU,/dt has not yet changed its sign. In that case, the time interval A (FIG. 2b) is measured from the moment the voltage U, begins to collapse (FIG. 2a, point A) up to the moment when said voltage U, (FIG. 2a, point 3,) exceeds the minimum.

The duration of the time interval A 1, depends upon the extent of damping and distortions in the fronts of the electromagnetic voltage waves emerging on the very line being protected as response to a short-circuit in said line, as they propagate from a short-circuit point to the beginning of the line.

It is apparent that in case of a short-circuit at the end of the very line 2 being protected (FIG. 3), the duration of the signal being shaped, proportional to the derivative dU,/dt as long as said derivative has not changed its sign (FIG. lb, interval A t,), will have a greater value than in case of less remote short-circuits on said line.

The provision of surge suppressors l and l (FIG. 3) connected in series with the very line 2 being protected in its beginning and end correspondingly, increases the extent of damping and distortions of said electromagnetic voltage waves emerging in said line 2 as response to faults beyondsaid surge suppressors l and l at the convertor substations (not shown on the figures).

Therefore the duration of a signal being shaped, proportional to said derivative dU,/dt (FIG. 2b), until said derivative changes its sign (FIG. 2b, interval A in case of a short-circuit beyond the surge suppressor I turns out to be essentially longer than the duration of said signal A t, (FIG. lb) in case of a short-circuit occurring in a most remote point on the very line being protected.

The difference in the duration between said time intervals A r, (FIG. lb) and A t, (FIG. 2b) remains practically constant regardless of a number of conversion bridges actuated at each of the convertor substations connected by said line 2 (FIG. 5) being protected, i.e., it does not actually depend upon the working voltage in the very line 2 being protected. Said difference depends upon the inductance of the surge suppressors 1 and 1' and the initial parameters of the very line 2 being protected, said parameters being taken practically constant for the given D.C. transmission line.

In order to establish the difference in the duration of said time intervals A 2 (FIG. 1b) and A 1 (FIG. 2b) and localize a fault, it is necessary to produce a reference signal at the moment of the emergence of a signal being shaped proportional to the derivative dU ldt of the voltage U (FIGS. 1a and 2a) in the beginning of the very line 2 being protected, the duration of said reference signal being selected longer than the time interval A t (FIG. lb) of said signal being shaped, until said derivative changes its sign in case of a short-circuit in the end of the very line 2 (FIG. 3) being protected, and shorter than the time interval A t (FIG. 2b) within said time interval (i.e., until said derivative changes its sign) in case of a short-circuit beyond the surge suppressor 1 (FIG. 3) at an invertor substation connected to the end of said line.

With such selection of the duration of the reference signal and the moment of its beginning, the duration of a signal being shaped and proportional to the derivative dU ldt of the voltage U in the beginning of the very line 2 being protected, which is shorter within the time interval until said derivative dU ldt changes its sign than the duration of said reference signal, will indicate that there is a short-circuit on said line being protected; in case the duration of said signal being shaped within the same time interval is longer than the duration of said reference signal, the fault is in fact a failure at the convertor substations connected by said line.

The device for the realization of said method of protecting D.C. transmission lines against short-circuiting comprises: a selection unit 4 connected to the very line 2 (FIG. 3) being protected with the aid of a voltage divider 3, one of the inputs 5 of a protection unit 6 being connected to an output of said selection unit 4 and the two other inputs 7 and 8 of the protection unit 6 being connected to said line 2 with the aid of a DC. transformer 9 and said voltage divider 3, correspondingly. The outputs 10 and 11 of said unit 6 are connected to the control devices (not shown on the figures) of a rectifying substation of said line, an input 10 of the protection unit 6 being connected to said control devices via a delay element 12 and an output 11 being coupled with said control devices directly.

The selection unit 4 is intended for controlling the operation of the protection unit 6. The former comprises: series-connected differentiator l3, amplifier 14, maximum pulse duration selector l5 fitted with a reference signal presetter I6, flip-flop 17 and a logical prohibitive circuit 18. Connected in series with said maximum duration selector is an actuator 19 whose output is at the same time an output of the selection unit 4 and connected to an input 5 of the protection unit 6.

The differentiator 13 serves to shape a signal proportional to a derivative dU,/dt (FIG. 1) of the voltage U, in the beginning of the very line 2 being protected (FIG. 3). Thus, a signal being shaped is a signal proportional to the time derivative dU ldt of the voltage U on the line 2 being protected.

The input of the differentiator 13 is an input of the selection unit 4 being connected, via the voltage divider 3, to the very line 2 being protected. The maximum duration selector 15 is intended for selection, out

of a signal being shaped dU /dt, preamplified by the amplifier 14, of those pulses whose duration longer than the duration of a reference signal preset by the presetter 16 of said selector 15.

One of the functions of the flip-flop 17 is to provide the selection by the selector 15 of only the initial of said pulses.

The flip-flop 17 has two inputs 20 and 21 and two outputs 22 and 23, the input 20 of said flip-flop 17 being connected to the input of the presetter 16, while the output 22 of said flip-flop 17 is connected to one of the inputs 24 of the logical prohibitive circuit 16. The second input 25 of the logical circuit 18 is connected to the output of the presetter 16. The output of the logical prohibitive circuit 18 is an output of the selector l5 and is connected to one of the inputs 26 of the actuator 19.

The logical prohibitive circuit 18 in the selector 15 ensures the comparison of the duration of pulses in the amplified signal being shaped, which is coming in to the input of the selector 15, with the duration of the reference signal preset by the presetter 16 of the reference duration signal.

The second input 27 of the actuator 19 is connected to the output 23 of the flip-flop 17, while the input 21 of said flip-flop 17 is coupled with the output of the delay circuit 28. The delay circuit 28 prepares the selection unit 4 for operation and its input is connected to the very line 2 being protected via the voltage divider 3.

The protection unit 6 is intended for finding faults in DC. transmission lines irrespective of their location: on the very line 2 being protected or at the convertor substations linked with said line.

The protection unit 6 comprises a minimum voltage member 29, a maximum current member 30 and logical circuits 31, 32 and 33, the circuit 32 being a prohibitive circuit, while the other circuits 31 and 33 being matching circuits.

The input of the minimum voltage member 29 is the input 8 of the protection unit 6 and is connected, via the voltage divider 3, to the very line 2 being protected, while the input of the maximum current member 30 is the input 7 of said unit 6 and is coupled with said line 2 by means of the DC. transformer 9.

The maximum current member 30 has two outputs 34 and 35, one of which, i.e., 34, is connected to one of the inputs 36 of the logical prohibitive circuit, while the other is connected to the input 37 of the logical matching circuit 31.

The output of the minimum voltage member 29 is connected at a time both to the second input 38 of the logical matching circuit 31 and to one of the inputs 39 of the logical matching circuit 33, while the second input 40 of said circuit 33 is linked with the output of the logical prohibitive circuit 32. The second input of said circuit 32 is the input 5 of the protection unit 6 and is coupled with the output of the selection unit 4.

The output of the logical matching circuit 31 is the output 10 of the protection unit 6 connected to the control devices of the rectifying substation via the time delay element 12, while the output of the logical circuit 33 is the output 11 of said unit 6, connected to the control devices of said rectifying substation directly.

Following furtheron is an exemplary embodiment of the functional schematic circuit diagram of the device, according to the invention. Thereon, a differentiator 13 (FIG. 4) is fashioned as a passive differentiating circuit arranged on a chain connection of two double-wound transformers 41 and 42. Connected to the output of the differentiator 13 is an amplifier 14 made on a transistor 43.

The primary winding of the transformer 41, which is at the same time the input of the differentiator 13 is connected to a tap 44 of the voltage divider 3. To produce a signal being shaped, of the derivative dU,/dt of only one sign, the secondary windings of the both transformers 41 and 42 are shunted by diodes 45 and 46, while a diode 47 is connected in series with the primary winding of the transformer 42.

The amplifier 14 is also intended for stabilizing the level of an output signal of the differentiating circuit. The presetter 16 of the reference duration signal is fashioned as a common-type univibrator made on transistors 48 and 49 and triggered by transistor 50.

Connected to the input 20 of a flip-flop 17 is a chain of series-connected diode 51 and capacitor 52. A prohibitive circuit 18 consists of two parallel-connected transistors 53 and 54, one of which, i.e., 53, is coupled by its base to the output of the univibrator and the other 54, to the output 22 of the flip-flop 17.

The actuator 19 of the selection unit 4 comprises a thyristor 55 and transistors 56 and 57. The transistor 56 is connected with its collector to the control circuit of said thyristor 55 via a chain made up of seriesconnected capacitor 58 and diode 59. The transistor 57 is meant for disconnecting said thyristor 55 when a signal from the output 22 of the flip-flop 17 is fed onto its base (the input 27).

The collector of the transistor 57, fashioned as an output circuit of the actuator 19 of the selection unit 4, is connected to the collector of a transistor 60, said collector being at the same time the input of the protection unit 6.

A maximum current member 30 is based on transistors 60 and 61 whose base circuits are connected, via a threshold element 62, to the output of a rectifier 63 coupled with a DC. transformer 9 via an intermediate transformer 64. To control the triggering threshold of the maximum current member 30, there is a variable resistor 65.

The minimum voltage member 29 is made to the circuit of comparison of the preset voltage of the divider 3 with the use of a threshold element '66 provided between the base of a transistor 67 and a variable resistor 68 connected to a tap 69 of the voltage divider 3. Said variable resistor 68 serves to vary the triggering voltage of the minimum voltage member 29.

Via a resistor 70, the collector of the resistor 60 of the maximum current member 30 is combined with the output of the minimum voltage member 29 into the matching circuit 33. When combined, the collector of the transistor 57 and the collector of the transistor 60 make up the prohibitive circuit 32.

The collector of the transistor 61 of the maximum current member 30 is combined, via a resistor 71, with the output of the minimum voltage member 29 into the matching circuit 31, while the resistor 71 is connected to the input of the delay element 12.

The voltage delay circuit 28 is fashioned as a chain RC formed by a resistor 72 and a capacitor 73 and connected to a tap 74 of the voltage divider 3. The common point of the resistor 72 and the capacitor 73 is connected to the input 21 of the flip-flop 17 via a threshold element 75, the flip-flop 17 comprising two transistors, 76 and 77 respectively.

The device for a selective protection of a DC. transmission line operates as follows.

With the connection of said D.C. transmission line, normally effected by cutting in a rectifying substation provided in the beginning of the very line 2 (FIG. 3) being protected, the voltage U on the line protected and the current in said line attain preset working values.

As this takes place, a voltage proportional to the working voltage on the very line 2 being protected emerges across the input of the delay circuit 28 (FIG. 3). After some time preset by the delay circuit 28, a short-time signal emerges across the output of said delay circuit 28, which is then fed onto the input 21 of the flip-flop 17 of the maximum duration selector 15. Under the action of said signal, the flip-flop 17 is set into such a steady state as would enable the switching of said flip-flop 17 into an opposite steady state by means of a signal fed onto the input 20 of said flip-flop.

At the moment of the flip-flop 17 switching by means of an output signal of the delay circuit 28, a short-time signal emerges across the output 23 of said flip-flop 17, said signal being then fed onto the input 27 of the actuator 19 of the selector 15. With the aid of the latter signal, the actuator 19 is readied to feed a signal onto the input 5 of the protection unit 6 as soon as the above short-time signal emerges at its input 26. Thus, the selection unit 4 is ready for operation in case of shortcircuits on the very line 2 being protected or at its convertor substations provided in it.

In case of a normal functioning of a DC. transmission line, there is no signal whatsoever at the output of the minimum voltage member 29 of the protection unit 6, as said member 29 triggers only as response to a drop in the voltage U on the very line 2 being protected to a value essentially lower than the working voltage U, on said line. At the same time, signals are present across the outputs 34 and 35 of the maximum current member 30, as said member 30 triggers at such a current in the very line 2 being protected, whose value smaller than the value of the minimum working current of said line.

In general, despite the presence of signals across the outputs 34 and 35 of the minimum current member 30, there are no signals across the outputs 10 and 1 1 of the protection unit 6 in case of a normal functioning of a DC. transmission line, as in this case no signal is fed from the output of the minimum voltage member 29 onto the input 38 of the logical matching circuit 31 and onto the input 39 of the logical matching circuit 33.

In case of a short-circuit on the very line 2 being protected or of some fault at a convertor substation, the voltage U, (FIGS. la and 2a) in the beginning of the very line 2 (FIG. 3) being protected tends to drop, while the current in said line shows no decrease, at least.

As this takes place, signals across the outputs 34 and 35 of the maximum current-member 30 survive and, the more so, a signal emerges at the output of the minimum voltage member 29 as said member 29 triggers as response to a drop in said voltage U From the output of the minimum voltage member 29 a signal is fed onto the input 38 of the logical matching circuit 31, while, across the input 37 of said circuit 31, there is a signal fed from the output 35 of the maximum current member 30. Once the both input signals are present, a signal emerges at the output of the logical circuit 31, i.e., at the output 10 of the protection unit 6. Said signal is used to trigger the delay element 12. If, with the elapse of the time period preset by the delay element 12, a fault on the DC. transmission line is not eliminated and a rectifying substation is not cut out by other means, a signal will emerge across the output of the delay element 12. This signal will go to the input of a control device of the rectifying substation, said control device cutting out said rectifying substation there A signal across the output 11 of the protection unit 6 will emerge on condition of the presence of signals across the inputs 39 and 40 of the logical matching circuit 38, which is, apparently, possible if there is no signal at the input of the logical prohibitive circuit 32, i.e., at the input of the protection unit 6, coupled with the output of the selection unit 4. In said instance, with the absence of a signal across the input of the logical prohibitive circuit 32 and the presence of signals across the outputs 34 and 35 of the maximum current member 30 and across the output of the minimum voltage member 29, there will emerge signals across the outputs of the both logical matching circuits 31 and 33, i.e., across the outputs and 11 of the protection unit 6, at a time. From the output 11 of the protection unit 6, the signal goes to the control devices of the rectifying substation directly, said rectifying substation being cut out without delay.

Thus, the operation of the protection unit 6 depends upon whether or not a signal is fed from the output of the selection unit 4 onto the input 5 of the protection unit 6.

The selection unit 4 operates as follows.

In case of a short-circuit on the very line 2 being protected, a signal being shaped emerges across the output of the differentiator 13, said signal being proportional to a derivative dU,/dt (FIG. 1b) of the voltage U (FIG. la) in the beginning of the very line 2 (FIG. 3) under protection. The signal being shaped is amplified by the amplifier 14 and then fed onto the input of the pulse maximum duration selector 15. At a moment corresponding to the beginning of a drop in the voltage U (FIG. la, point A), i.e., to the moment of the emergence of the signal being shaped, a presetter 16 of the reference duration signal is triggered, which feeds a reference duration signal onto the input 25 of the logical prohibitive circuit 18, beginning from a given moment of time. The duration of said reference signal, is preset to be shorter than the least duration A t, (FIG. 2b) of the signal being shaped, until the derivative dU,/dt changes its sign in case of a short-circuit beyond a surge suppressor 1 (FIG. 3) at an invertor substation, and longer than the maximum duration A t, (FIG. 1b) of the signal within said time interval in case of a short-circuit in the end of the very line 2 (FIG. 3) being protected.

At the moment of collapsing of the initial pulse, in a signal being shaped, which was amplified by the amplifier 14 (FIG. la, point B), the flip-flop 17 corresponding to the end of the time interval A t, wherein the derivative a U dt has not yet changed its sign, switches over, under the action of a signal fed onto the input 20 (FIG. 3) of said flip-flop 17, from its original steady state into another steady state, which conditions the emergence on its output 22 of a short-time signal coinciding in time with themoment of the ending of the time interval A t, (FIG. 1b).

From the output 22 (FIG. 3) of the flip-flop 17, said signal goes to the input 24 of the prohibitive circuit 18.

As in case of a short-circuit on the very line 2 being protected the time interval A I (FIG. lb) shorter than the duration of the reference signal of the presetter 16, its signal across the input 24 of the prohibitive circuit 18 is effective only during the presence of a signal at the input 25 of said circuit 18. The signal across the input 25 of the prohibitive circuit 18 is an obstruction to the signal effective at the input 24 of said circuit 18 in being fed to the input 26 of the actuator 19 of the selection unit 4.

Thus, in case of short-circuits on the very line 2 being protected, a signal from the output of the selection unit 4 is not fed onto the input 5 of the protection unit, which ensures the passage of a signal from the output 34 of the maximum current member 30 onto the input 40 of the logical matching circuit 33 via the prohibitive circuit 32.

Consequently, in case of a short-circuit on the very line 2 being protected, signals are present across the inputs 37, 38, 39, and 40 of the matching circuits 31 and 33, which ensures the availability of signals across the outputs of the both said circuits 31 and 33, i.e., across the outputs l0 and 11 of the protection unit 6.

In case of faults on the invertor substation, which call for cutting out the rectifying substation (e.g., in case of a short-circuit beyond the surge suppressor 1) for their elimination, the duration A t, (FIG. 2b) of a signal being shaped, until the derivative dU ldt (FIG. 2b) of the voltage U, has changed its sign, longer than the duration of the reference signal of the presetter 16 (FIG. 3); therefore, the switching of the flip-flop 17 into another steady state by a signal emerging across the input 20 of said flip-flop 17 at the moment of the collapse of the first pulse in the signal being shaped now occurs after the complete elapse of the duration of the reference signal of the presetter 16.

Consequently, a short-time signal emerging, in the course of said switching of the flip-flop 17, across its output 22 will be now effective across the input 24 of the prohibitive circuit 18 in the absence of the reference signal across the input 25 of said circuit 18.

Thus, said short-time signal operating across the input 24 of the prohibitive circuit 18 will be fed onto the input 26 of the actuator 19. Under the action of said signal the actuator 19 shapes an output signal of long duration, which is fed onto the input the prohibitive circuit 32 of the protection unit 6 and precludes the passage of a signal from the output 34 of the maximum current member 30 to the input 40 of the matching circuit 33.

The absence of a signal across one of the inputs (40) of said circuit 33 precludes the emergence of a signal across the output of said matching circuit 33, i.e.,

across the output 11 of the protection unit 6. At the same time, there will be a signal across the output of the matching circuit 31, as signals will be present on the both inputs 37 and 38 of said circuit, obtained respectively from the output 35 of the maximum current member 30 and the output of the minimum voltage member 29.

Thus, in case of faults at the invertor substation, a signal will emerge only across the output 10 of the protection unit 6, and the cutting-out of the rectifying substation will be effective with a certain delay preset by the delay element 12.

The provision of the flip-flop 17 in the selector l6 helps measure only the initial pulse in the signal being shaped, as after the flip-flop 17 switching under the action of a signal emerging at its input 20 virtually at the moment of the collapse of said initial pulse, said flipflop 17 does not return to its former steady state but remains in the other steady state and, consequently, is unable to switch again, for instance, at a time interval corresponding to the collapse of the second pulse in said signal being shaped.

The switching of the flip-flop 17 into a state conducive to its repeated switch-over is possible only with a repeated connection of the DC transmission line after its cutting-out by the control devices of the rectifying substation. This is done by a signal emerging across the output of the delay circuit 20 and fed onto the input 21 of the flip-flop 17, thus switching the latter; at 'a time, a short-time signal emerges across the output 23 of said flip-flop 17, which is then fed onto the input 27 of the actuator 19.

This latter signal is made use of to discontinue the output signal of the actuator 19, that emerged in the course of a fault at the invertor substation.

Said switching of the flip-flop l7 and collapse of the output signal of the actuator 19, occurring during the cutting-in of the rectifying substation after its disconnection, make the selection unit 4 and the device as a whole ready for another cycle. Now follows the de-' scription of the operation of a device embodied to a functional schematic circuit diagram as on FIG. 4.

In case of a fault at the convertor substation, a transient occurs in D.C. transmission, which is accompanied by a corresponding variation in the voltage U, (FIG. 1a) on the line being protected. Varying proportionately to said voltage U, in the beginning of the line is also the current in the voltage divider 3 (FIG. 4), which is flowing around the primary winding of the transformer 41 of the differentiator 13. As a result, a

signal being shaped emerges in the secondary winding of the transformer 42, proportional to the time derivative dU,/a't (FIG. 2) of the voltage U, in the beginning of the line being protected.

On passing through the amplifier 14 (FIG. 4), a signal of the derivative dU,/dt, shaped as a square-shaped pulse, is fed onto the base of a triggering transistor of the univibrator, as well as onto the input 20 of the flip-flop 17.

Under the action of the forward front of the initial pulse of the signal being shaped, the univibrator is triggered; as a result, the transistor 53 of the prohibitive circuit 18 which is normally in position closed, changes over into position opened for a time determined by a reference duration signal shaped by said univibrator being, at a time, the presetter 16 of the reference duration signal. I

Under the action of the rear front of the initial pulse of the signal being shaped, the flip-flop switches over from its original position at which the transistor 77 is opened and the transistor 76 closed into its opposite steady state At the moment of the flip-flop 17 switching, a pulse of positive polarity is fed from output 22 onto the base of the transistor 54 of the prohibitive circuit 18 as a result of which said transistor 54 passes 12 over from its normal state opened into state closed *for a time A (FIG. 2b).

Thus, in the course of establishing the duration of the initial pulse of a signal being shaped of the derivative dU,/dt of the voltage U, in the beginning ofa linebeing protected, two pulses one of reference duration and a short-time pulse emerging at the moment of the ending of the interval A t (FIG. 2b) are fed onto the inputs 24 and 25 of the prohibitive circuit 18. A signal will emerge across the output the collector of the transistor 53 (FIG. 4) of the circuit 18, because said transistor 53 will return to position closed earlier than the transistor 54 at the moment of the elapse of the time interval A t, and, for a short time, will pass over into position closed, as a result of which the transistor 56 of the actuator 19 will pass over into position opened, thus ensuring the emergence of an output signal of the selection unit 4.

Consequently, a signal should arise at the output of the prohibitive circuit 18 and, naturally, at the output of the selection unit 4, because the duration A t (FIG. 2b) of the initial pulse of the signal being shaped of the derivative dU,/dt of the voltage U, in the beginning of the line under protection will turn out to be longer than the duration of the reference duration signal.

The single-action basis of the flip-flop 17 (FIG. 4) in case of a fault precludes the possibility of the emergence of a signal across its output 22 at the moments of the collapse of pulses following the initial one in the signal being shaped of the derivative dU,/dt; as a result, there is no room for a repeated signal across the output of the prohibitive circuit 18 in case of one and the same fault.

A drop in the voltage U, in the beginning of the line 2 being protected results in voltage dropping in a tap 69 of the voltage divider 3. With a reduction in the voltage in the tap 69 below a certain value determined by the threshold element 66, the transistor 67 passes over from position closed into position opened. If, in this case, the current in the line under protection will be bigger than a certain minimum value, the voltage in the resistor 65 will exceed the voltage of triggering the threshold element 62, and the transistors 61 and 60 will be in position closed. The potential across the collector of the transistor 61 will be essentially lower than the potential across the collector of the transistor 67, which will bring about a drop in the voltage of a preset sign across the resistor 71 at the output of the "matching circuit 31, i.e., the emergence of a signal across the output 10 of the protection unit 6. The potential across the collector of the transistor 60 of the maximum current 30 depends not only on the value of the current in the transmission line but also on the condition of the thyristor 55 of the actuator 19 of the selection unit 4.

In the case under consideration, when there is a fault in the convertor substation, the actuator 19 is in a triggered position (the thyristor 55 is opened); as a result, the potential across the collector of the transistor 60 difiers little from the potential across the collector of the transistor 67. Hence, there is no signal across the output resistor 70 of the matching circuit 33, i.e., such signal is absent at the output 11 of the protection unit 6.

Thus, in case of faults in the convertor substation, the disconnection of a transmission line is effected with a delay preset by the delay element 12 connected to the output of the protection unit 6.

With the rehabilitation of the voltage on the line under protection in case of a successful repeated connection of the transmission line, a short-time signal will emerge at the output of the delay circuit 28, said signal bringing the flip-flop 17 back into the original steady state. As this takes place, the transistor 76 of the flip-flop 17 will pass over into position closed, with the resultant emergence of a signal of negative polarity across the base of the transistor 57 of the actuator 19, and the transistor 57 will pass for a short time, over into position opened. As a result, the thyristor 55 will come back into its original position closed, thus preparing the device for repeated action.

In case of a fault on the very line 2 being protected (FIG. 1), the duration A t (FIG. 1b) of the initial pulse of the signal being shaped of the voltage U shorter than the duration of the reference duration signal being shaped by the presetter 16 (FIG. 4) of the reference duration signal. Consequently, the switching of the flipflop 17, which corresponds to the elapse of the time interval A 1 (FIG. 1b) will occur earlier than the duration of the reference duration signal expires. That means that the transistor 54 of the prohibitive circuit 18 will change its state for closed for a short period of time at the moment of the elapsing of the time interval A t (FIG. lb) and will have returned to position opened prior to the transistor 53 (FIG. 4) of the prohibitive circuit 18 switching from position opened into position closed, which precludes the emergence of a signal across the input 26 of the actuator 19. As a result, there will emerge no signal across the output of the selection unit 4 either. The thyristor 55 of the actuator 19 will remain in position closed, and the potential across the collector of the transistor 60 will not differ from the potential across the collector of the transistor 61, which conditions the drops in the voltages of preset signs across the both resistors 70 and 71, i.e., the emergence of signals on the both outputs 10 and 11 of the protection unit 6.

Thus, in case of a fault on the very line 2 being protected, an output signal emerges on either of the outputs 10 and 11 of the protection unit 6, and the fault is eliminated by cutting out power transmission without delay throughout the circuit of the output 11 of said protection unit 6.

In case of a fault at the convertor substation, an output signal will emerge only across one of the outputs 10 of the protection unit 6, and the fault is eliminated by cutting out power transmission with delay be means of the delay element 12 throughout the circuit of the output 10 of said protection unit 6.

Thus, the present invention stands out for a higher sensitivity, as compared with the known devices, which does not depend upon the number of conversion bridges provided in a DC. transmission line, and per mits to distinctly differentiate between faults at convertor substations from faults on a very line being protected, whether said line is the cable one, aerial, or has a cable link.

What we claim is l. A method of protecting D.C. transmission lines,

which provides for shaping a signal proportional to a time derivative of a varied voltage on said line being protected; presetting a reference signal at the moment of the emergence of said signal being shaped, whose duration is selected to be longer than the maximum duration of said signal being shaped within a time interval until said derivative changes its sign in case of shortcircuits on the very line being protected, and shorter than the minimum duration of said signal being shaped within said time interval in case of faults at conversion substations on said line; comparing the duration of said signal being shaped within a time interval, until said derivative changes its sign, with the duration of said reference signal, the duration of said shaped signal shorter within the time interval than the duration of said reference signal indicating a likely short-circuit on the very line being protected while the duration of said shaped signal longer within the same time interval than the duration of said reference signal indicating a likely fault at a convertor substation on said D.C. transmission line.

2. A device for protecting D.C. transmission lines comprising: a voltage divider connected to a DC. transmission line being protected, a selection unit connected to said voltage divider and made up of a differentiator, an amplifier series-connected to said differentiator a pulse maximum duration selector seriesconnected to said amplifier and having a reference sig nal presetter, a flip-flop one of whose outputs is connected to an input of said reference signal presetter, a logical prohibitive circuit, one of whose inputs is connectedto the output of said flip-flop and the other, to the output of said reference signal presetter; an actuator one of whose inputs is connected to the output of said logical prohibitive circuit of said selection unit and the other, to the other output of said flip-flop of the same unit; a delay circuit whose input is connected to said voltage divider and the output, to the other input of the flip-flop; a DC. transformer connected to the line being protected; a protection unit having four inputs, two of which being connected to said transformer, the third one coupled with said actuator of said selection unit and the forth one, to said voltage divider, said protection unit being made up of a maximum current member whose inputs are the first two inputs of said protection unit; a logical prohibitive circuit one of whose inputs is the third input of said unit and the other input is connected to the output of said maximum current member; a minimum voltage member whose input is the forth input of said unit; a first logical matching circuit one of whose inputs is connected to the output of said maximum current member and the other input, to the output of said minimum voltage member, the output of this logical circuit being the first output of the entire protection unit; a second logical matching circuit one of whose inputs is connected to the output of said minimum voltage member and the other input, to the output of said prohibitive circuit, the output of this matching circuit being the second output of the entire protection unit; a delay element whose input is connected to the first output of said protection unit. 

1. A method of protecting D.C. transmission lines, which provides for shaping a signal proportional to a time derivative of a varied voltage on said line being protected; presetting a reference signal at the moment of the emergence of said signal being shaped, whose duration is selected to be longer than the maximum duration of said signal being shaped within a time interval until said derivative changes its sign in case of shortcircuits on the very line being protected, and shorter than the minimum duration of said signal being shaped within said time interval in case of faults at conversion substations on said line; comparing the duration of said signal being shaped within a time interval, until said derivative changes its sign, with the duration of said reference signal, the duration of said shaped signal shorter within the time interval than the duration of said reference signal indicating a likely short-circuit on the very line being protected while the duration of said shaped signal longer within the same time interval than the duration of said reference signal indicating a likely fault at a convertor substation on said D.C. transmission line.
 2. A device for protecting D.C. transmission lines comprising: a voltage divider connected to a D.C. transmission line being protected, a selection unit connected to said voltage divider and made up of a differentiator, an amplifier series-connected to said differentiator a pulse maximum duration selector series-connected to said amplifier and having a reference signal presetter, a flip-flop one of whose outputs is connected to an input of said reference signal presetter, a logical ''prohibitive'' circuit, one of whose inputs is connected to the output of said flip-flop and the other, to the output of said reference signal presetter; an actuator one of whose inputs is connected to the output of said logical ''prohibitive'' circuit of said selection unit and the other, to the other output of said flip-flop of the same unit; a delay circuit whose input is connected to said voltage divider and the output, to the other input of the flip-flop; a D.C. transformer connected to the line being protected; a protection unit having four inputs, two of which being connected to said transformer, the third one coupled with said actuator of said selection unit and the forth one, to said voltage divider, said protection unit being made up of a maximum current member whose inputs are the first two inputs of said protection unit; a logical ''prohibitive'' circuit one of whose inputs is the third input of said unit and the other input is connected to the output of said maximum current member; a minimum voltage member whose input is the forth input of said unit; a first logical ''matching'' circuit one of whose inputs iS connected to the output of said maximum current member and the other input, to the output of said minimum voltage member, the output of this logical circuit being the first output of the entire protection unit; a second logical ''matching'' circuit one of whose inputs is connected to the output of said minimum voltage member and the other input, to the output of said ''prohibitive'' circuit, the output of this ''matching'' circuit being the second output of the entire protection unit; a delay element whose input is connected to the first output of said protection unit. 